1. Field of the Invention
The present invention relates to a process for obtaining alkali metal or alkaline earth metal salts of tetracarbonylcobalt hydride and solutions thereof.
2. Description of the Prior Art
The alkali metal or alkaline earth metal cobalttetracarbonylates which correspond to the formula M[Co(CO).sub.4 ].sub.n in which M denotes an alkali metal or alkaline earth metal and n is 1 or 2 are known compounds employed as catalysts for numerous reactions involving carbon monoxide. Thus, for example, the alkali metal or alkaline earth metal cobalttetracarbonylates are employed as catalysts for carbonylation of alkyl halides, cycloalkyl halides or aryl halides for the preparation of organic acids or derivatives thereof, particularly esters. Hence, French Pat. No. 1,313,360 described a process for preparing derivatives of carboxylic acids, and particularly esters, by carbonylation of an organic halide with carbon monoxide in the presence of salts such as tetracarbonylcobalt hydride and in particular of alkali metal or alkaline earth metal salts of such hydride and of compounds having labile hydrogen atoms (water, alcohols, mercaptans, and primary or secondary amines) in a basic environment. This process is most particularly suitable for the preparation of phenylacetic acids and derivatives thereof. In French patent application No. 70/26,593, published under No. 2,055,331, it has been proposed to prepare phenylacetic acid by carbonylation of benzyl chloride with carbon monoxide in the presence of calcium cobalttetracarbonylate prepared by reacting a cobalt salt with carbon monoxide in the presence of lime, an iron/manganese alloy and sulfur-containing promoters, in a medium of water and methanol. An analogous process has been described for the preparation of .alpha.-thienylacetic acid (cf. French patent application No. 78/07,980 published, under No. 2,420,534). Finally, in French patent application No. 75/00,533, published under No. 2,297,200, and the Addition thereto, No. 75/29,459, a process was proposed for double carbonylation of arylmethyl halides to arylpyruvic acid with carbon monoxide in the presence of an alkaline earth base and of an alkali metal salt or alkaline earth metal salt of tetracarbonylcobalt hydride, under a carbon monoxide pressure of between 5 and 200 bars. Alternative versions of this process have been described in U.S. Pat. No. 4,351,952 and in French patent application No. 79/16,170 published under No. 2,429,772.
Other than their use as catalysts for carbonylation, the alkali metal or alkaline earth metal cobalttetracarbonylates can also be employed for the preparation of dicobaltoctacarbonyl by decomposition of tetracarbonylcobalt hydride liberated by acidifying solutions of alkali metal or alkaline earth metal cobalttetracarbonylates. They also serve as intermediates for the preparation of other metal salts, such as cobalttetracarbonylates of silver, zinc, mercury and cadmium which are employed as catalysts for carbonylation of alcohols to esters (cf. British Pat. No. 715,515). Finally, alkali metal or alkaline earth metal cobalttetracarbonylates are of considerable interest in industrial organic synthesis and for this reason it would be desirable to develop a process for preparing these compounds which could be easily employed on an industrial scale. Whatever the destination of the alkali metal or alkaline earth metal cobalttetracarbonylates (preparation of dicobaltoctacarbonyl or direct use as a catalyst in the above-mentioned carbonylation reactions), serious need exists in the industry for a process for preparing alkali metal or alkaline earth metal cobalttetracarbonylates starting from common inorganic salts of cobalt (II) and carbon monoxide, at ambient pressure or under a moderate pressure of carbon monoxide and providing, if necessary, solutions of alkali metal or alkaline earth metal cobalttetracarbonylates which can be employed directly in industry for use in carbonylation processes. Numerous methods have been suggested without providing a satisfactory solution to this problem. Thus, as early as 1936, G. W. Coleman et al, J. Am. Chem. Soc., 58, beginning at page 2160, described a process for preparing potassium cobalttetracarbonylate by the reaction of cobalt nitrate with carbon monoxide at normal pressure in the presence of various additives promoting the absorption of carbon monoxide, such as potassium cyanide, cysteine or tartaric acid. The use of potassium cyanide has been pursued by numerous authors, for example: A. A. Blanchard et al, J. Am. Chem. Soc., 62, 1192-1193 (1940); F. Seel, Z. Anorg. Chem., 269, 40-42 (1952); Y. Takegami et al, Bull. Chem. Soc. Jap., 37, 181-182 (1964); R. J. Clark et al, J. Organomet. Chem., 11, 637-640 (1968). Although this process has the advantage of being capable of operation at atmospheric pressure, it cannot be employed industrially on account of the low reaction rate and/or low yields. In turn, W. Hieber et al, Z. Anorg. Chem., 269, 308-316 (1952), have proposed an improved laboratory process for the synthesis of mercury and silver cobalttetracarbonylates involving an intermediate preparation of the tetracarbonylate ion Co(CO).sub.4.sup.- by reacting carbon monoxide at normal pressure with cobalt nitrate hexahydrate and sodium dithionite (Na.sub.2 S.sub.2 O.sub.4) in an aqueous solution of ammonia. By reason of the conditions employed, this process is of interest only for the laboratory preparation of silver and mercury cobalttetracarbonylates. In fact, the production of the cobalttetracarbonylate ion Co(CO).sub.4.sup.- depends on the use of a stoichiometric quantity of sodium dithionite which acts as a reducing agent for the cobalt [cf. W. Hieber et al, Z. Anorg. Chem., 269, 292-307 (1952)]. Moreover, the ammoniacal solutions of ammonium cobalttetracarbonylate which are thus obtained cannot be directly used for certain of the above-mentioned carbonylation reactions, such as the preparation of thienylacetic, phenylacetic or phenylpyruvic acids, which require the use of alkali metal bases or alkaline earth metal bases in an aqueous medium or aqueous alcohol medium. To overcome the specific disadvantages of the process of Hieber et al, loc. cit., it has been proposed in French Patents and Patent Applications No. 70/26,593, No. 78/7,980 and No. 73/29,840 to prepare aqueous, alcoholic or aqueous alcoholic solutions of alkali metal or alkaline earth metal cobalttetracarbonylates by reacting a cobalt salt (chloride, sulfide or bromide) with carbon monoxide, in an alcoholic or aqueous alcoholic medium, in the presence of an alkali metal hydroxide or alkaline earth metal hydroxide, a catalytic amount of a sulfur compound (sodium dithionite, sodium sulfide and sodium thiosulfate) and an iron/manganese alloy. The principal disadvantage of this process is the use of an additional component, the iron/manganese alloy, the use of which in the form of powder complicates the process. Furthermore, the production of good results depends on the use of large quantities of manganese, on the order of 1 gram-atom of metal per mole of cobalt salt, which makes this process unattractive from the point of view of economics.
Finally, none of the processes for preparing cobalttetracarbonylates which are described in the prior art has satisfactorily solved the problem of the industrial preparation of alkali metal or alkaline earth metal cobalttetracarbonylates.